It is well-known in the art to provide photovoltaic cells for various uses, particularly in generating electricity from solar energy. Such cells are typically quite small and must be electrically connected in larger grids or modules for common electrical applications. The conventional procedure for producing such photovoltaic modules is to coat a portion of both sides of each photovoltaic cell with a conductive metal or metal alloy to form an electrical contact. Electrical wires are subsequently soldered to the electrical contacts of a group of such coated cells in order to form a larger, interconnected cell grid in which the cells are connected in series or parallel relationship. Thick film coatings comprised of (1) silver or silver alloys containing small amounts of glass or (2) aluminum are commonly used as the electrical contacts for photovoltaic cells. In some solar cell production methods, the rear contact of the solar cell is made of aluminum but has "windows" or openings that expose the underlying silicon. Those openings are filled with a silver/glass coating which bonds to the silicon substrate and makes electrical contact with the aluminum coating. The segments of silver coating that fill the windows are known as "soldering pads", since it is far easier and more beneficial to bond copper conductors to the soldering pads than it is to bond them directly to the aluminum layer.
In both manufacturing and in many common applications of photovoltaic modules, the cells are subjected to continuous high temperatures or else to thermal cycles at regular or irregular intervals. For example, in the ethylene vinyl acetate (EVA) lamination procedure that typically follows cell stringing (interconnecting) in the manufacture of multi-cell modules, the cells are subjected to temperatures as high as about 150.degree. C. for about 45-60 minutes. When used in the production of solar energy, the cells will heat up during a cycle of exposure to sunlight and then cool down again to ambient temperatures at night. In other applications, the heating and cooling cycles may be much more frequent. Accordingly, an important characteristic of such cells is their ability to withstand thermal aging, particularly with respect to their solder connections.
Prior art photovoltaic cells incorporating silver thick film electrical contacts or solder pads and conventional electrical solders applied by conventional dip or wave soldering techniques commonly show poor mechanical reliability of their solder bonds when subjected to thermal aging. Specifically, the strength of bonds made to silver/glass thick films on silicon using 63% tin/37% lead or 62% tin/36% lead/2% silver solders degrades by more than 80% upon exposure to temperatures of 150.degree. C. for one hour. Since exposure to such temperatures for such periods of time is typically required to bond the cells to glass to manufacture photovoltaic modules, the bonds in modules made with such solders are inherently weak. It is expected that further degradation in bond strength will continue at normal operating temperatures of photovoltaic modules. Stress testing of modules made in this manner indicate that their performance degrades relatively rapidly under conditions that produce mechanical loading on the module, including changes in temperature which can be expected to occur in typical applications.
The problem of thermal degradation of soldered thick film silver-bearing conductors in semiconductor devices has been reported and discussed in the literature. For example, in "Progress in and Technology of Low Cost Silver Containing Thick Film Conductors", by B. E. Taylor, J. J. Felten, and J. R. Larry, in Proceedings of the 30th Electronic Components Conference, New York, IEEE, 1980, pp. 149-166, the authors reported that such degradation on miniaturized and hybrid circuits on ceramic substrates can be reduced, in the case of silver/palladium thick films, by the use of 95% tin/5% silver solders instead of conventional tin/lead solders. But, because this solder has a particularly high melting point, its tin component tends to dissolve silver from the thick film during bond formation Accordingly, this procedure would not be useful for soldering thick films consisting of primarily silver, which are considerably less expensive than the silver/palladium thick films of the reference.
In a more recent reference, "The Thermal-Cycled Adhesion Strength of Soldered Thick Film Silver-Bearing Conductors", by C. R. S. Needes and J. P. Brown, in Proceedings of ISHM '89, ISHM, 1989, pp. 211-219, the authors concluded (at p. 215): "For the silver conductor, the best [thermal]results were obtained with [a solder of]10 Sn/88 Pb/2 Ag." FIG. VII at p. 215 of the ISHM reference clearly shows that, with silver thick films on alumina substrates, a solder comprising 10% tin, 88% lead and 2% silver demonstrated superior thermal-cycled adhesion strength as compared with a solder of 96% tin and 4% silver. The 96 Sn/4 Ag solder began to show a marked deterioration after fewer than 200 thermal cycles, whereas solder bonds prepared using the 10 Sn/88 Pb/2 Ag solder did not show significant deterioration until approximately 500-600 thermal cycles. Both of the foregoing references are incorporated herein by reference.
The literature in this field thus teaches away from the use of tin/silver solders containing predominant proportions of tin on silver or silver alloy thick film contacts or soldering pads for photovoltaic cells. The literature suggests that the problem may be caused, at least in part, by the tendency of the tin/silver solder to cause "leaching" or "scavenging" of silver, i.e. the dissolution of the silver thick film in the molten solder at elevated temperatures. The relatively high cost of silver as well as technical considerations make it impractical or uneconomical to use thicker silver films or solder with higher proportions of silver to try to overcome the leaching problem. As noted above, silver/palladium thick films are even more expensive than using silver alone. Moreover, the toxicity of lead limits the utility of the tin/lead/silver solders touted by the Needes and Brown reference cited above.
The literature in this field also suggests that the problem of thermal instability may be caused, at least in part, by the formation of intermetallic compounds between tin and silver because such compounds are known to be brittle and weak. These compounds can form not only while the solder is molten, but any time the solder is at an elevated temperature. Some formation of these compounds occurs slowly even at room temperature. It has been suggested that these intermetallic compounds are the result of tin from the solder diffusing through the glass and metal surface phases. These compounds, being brittle, cause the metallization to feral at low stress levels.
It is possible, of course, that both the silver scavenging and intermetallic compound mechanisms are involved in the thermal instability problem. This makes a solution to the problem even more difficult to predict because a solution to one of these mechanism may exacerbate the other.
For example, one researcher reported reducing the problem of intermetallic compound formation by using a high tin-bearing solder, such as 96% tin/4% silver. The explanation was that a 96 Sn/4 Ag solder has a melting point significantly higher than a solder consisting of 62 % tin, 36% lead, and 2% silver, resulting in bonding the tin more strongly to the solder and thereby reducing diffusion. On the other hand, data reported in the Needes and Brown reference cited above suggest that the high tin content and high melting point of a 96 Sn/4 Ag solder may lead to increased thermal degradation of solder bonds due to the silver scavenging mechanism. Accordingly, the prior art provides no clear solution to the problem of thermal aging of solder bonds to silver metallized photovoltaic cells.
These and other drawbacks of the prior art are overcome with the present invention.